Many men think that business casual dress includes a t-shirt. Unfortunately at many companies, t-shirts are considered a bit too casual even on casual day. Instead, choose a solid colour polo shirt, a band collar or button-down collar shirt. Camp style shirts also work well but should be tucked in for a professional look. During cooler weather, a light-weight mock or turtle neck sweater look professional. These will give you a comfortable casual look without being too casual.